AS923 needs initial DownlinkDwellTime to be set, because of RX DR impact

[terrillmoore]

See section 2.8.7 of the regional plan:

• Case DownlinkDwellTime = 0 (No limit): MinDR = 0 1243
• Case DownlinkDwellTime = 1 (400ms): MinDR = 2

What they don't say is that the device must initially assume DownlinkDwellTime of 1, which is critical, as the gateway might hav RX1DROffset > 0; in that case, a device that sends on DR2 must receive at DR2, not DR1 or DR0 (as suggested by the offset).

[terrillmoore]

Otherwise test 3.1 on the RWC 5020A AS923 1.1 test will fail (no downlinks received).

[terrillmoore]

Fixed by #452